1. Field of the Invention
This invention relates to an induction heating apparatus, and more particularly to such an apparatus in which the thermal deformation of a cover plate is absorbed so as to decrease the change in distance between an exciter and a heated element caused by the thermal expansion.
Even more particularly, the induction heating apparatus of the present invention finds effective use as an induction heating cooking apparatus (cooker), and accordingly, the invention will be illustrated and described with respect to an induction heating cooking apparatus.
2. Description of the Prior Art
The principle of induction heating apparatus is to generate heat by applying electric power of the standard line frequency to an exciter in the body of the apparatus, for example a cooking pot. An eddy current is induced at the bottom of the cooking pot above the exciter by the alternating magnetic field generated from the exciter. Heat is generated mainly by the eddy current and the electric resistance at the bottom of the cooking pot.
In such apparatus of the prior art an exciter is normally exposed so as to put the cooking pot in direct contact with the exciter. Such apparatus has disadvantages in unsightly appearance as a household apparatus and also is dangerous when water or other contents of the pot overflows.
Accordingly, consideration has been given to placing a flat cover plate facing the cookpot above the exciter, thereby protecting the exciter and improving the appearance and cleanliness of the apparatus. As a cover plate, it is preferable in practical application to use a plate made of non-magnetic metal having a relatively high electric resistivity, such as stainless steel. It is also preferable to use a thin cover plate for increasing the efficiency of the induction heating apparatus. However, when a flat thin stainless steel plate is used, the stainless steel cover plate is heated to cause thermal expansion and subsequent thermal deformation of the cover plate.
The thermal expansion of the cover plate is caused by the transmission of a part of the heat at the bottom of the cooking pot resulting from the induction heating of the cooking pot by the exciter, so as to heat the cover plate, whereby the temperature of the cover plate in contact with the cookpot will be higher than that of the non-contacting portion of the cover plate. Due to the temperature gradient produced thereby the part of the cover plate in contact with the cooking pot causes a higher thermal expansion that the non-contacting part of the cover plate, whereby a thermal buckling is caused and the central region in contact with the cooking pot is upwardly or downwardly curved and deformed. As a result, the cookpot is undesirably separated from the exciter to decrease the efficiency of the induction heating apparatus and to increase the vibration and noise because of the unstable condition of the cooking pot.
In general, the thermal stress and thermal deformation are related to each other in that when the thermal deformation is decreased, the thermal stress is increased. However, in accordance with this invention, the thermal deformation of the cover plate will be absorbed so as to minimize the distance between the cooking pot and the exciter. The apparatus is safe while reducing vibration and noise, even though the central part of the cover plate still expands by heat generated during cooking.